The invention relates to a system for positioning a cone part for a CRT, the cone part having a tapered and a wide end.
The invention further relates to a method of manufacturing a CRT comprising a method step in which a cone part having a tapered end and a wide end of or for a CRT is positioned in a positioning system.
Cathode ray tubes (CRTs) are used in display devices such as television apparatuses, computer monitors and radar devices.
A CRT comprises a glass envelope with a cone part. During manufacture of the CRT, the cone part is treated in the different stages of manufacture of the CRT, for instance, when the sealing edge is made and when the front part and the cone part are attached to each other, when an electron gun is provided in a neck part of the CRT and when reference areas are provided on the cone part. In these method steps, the cone part is positioned in a positioning system by which the cone part is centered, i.e. its position is controlled and determined to within a certain accuracy. The cone part has a tapered end and a wide end. The conventional positioning system comprises three positioning means which form three corners of a triangle having sides of equal length. The positioning means are conventionally ball-shaped. The three positioning means form a reference frame for handling the cone. The more accurate the reference frame, the better the position of reference areas, sealing edge and display window and electron gun can be controlled with respect to each other. A better control of such positions enhances, amongst others, the location of the electron beams in operation and the quality of the displayed image.
A conventional CRT comprises a glass envelope with a cone part having a tapered end adjoining a neck part. This neck part accommodates an electron gun for generating one or more electron beams. Deflection means for deflecting the electron beam(s) surrounds this tapered end and the neck part. A display window is attached to a wide part of the cone. There is an ever greater tendency and desire to enhance the image quality, increase the flatness of the display window of the CRT and decrease the depth and weight of the CRT. These tendencies and desires exclude each other to some extend and a compromise has to be found. Conventional CRTs have a round cone part, i.e. the tapered end of the cone part is substantially circular in cross-section. To find a better compromise, the tapered end of the cone is preferably manufactured in such a way that it has a substantially rectangular cross-section. For such a cross-section, the (coils of the) deflection means can, on average, be brought closer to the electron beams, allowing a better control of the deflection and location of the electron beams and resulting in an increase of the quality of the image and a decrease of the energy needed to deflect the electron beams. However the inventors have found that, when a conventional positioning system is used for a cone part having a tapered end a substantially rectangular cross-section, the accuracy of determining the position of such a cone part leaves much to be desired. The inaccuracies are substantially larger than for cone parts having a tapered end with a substantially circular cross-section. This decrease of the positioning accuracy reduces or may even eliminate the positive effects of using a cone part with a substantially rectangular cross-section. For simplicity xe2x80x98a cone part having a tapered end with a substantially rectangular cross-sectionxe2x80x99 is hereinafter sometimes also referred to as xe2x80x98a rectangular cone partxe2x80x99.
It is therefore an object of the invention to improve the accuracy with which the cone part is positioned and thereby the eventual quality of the CRT and/or to reduce the weight of the CRT and/or decrease the time needed for centering the cone.
To this end, the positioning system in accordance with the invention is characterized in that the system comprises a first and a second positioning means at positions substantially corresponding to two corners of a side of a rectangle, and a third positioning means at a position substantially corresponding to the middle of a side of the rectangle opposite the above-mentioned side, the first and second means having a contact surface which is convex in a direction from the tapered end to the wide end and concave towards a central part of the rectangle, the third positioning means having a contact surface which is convex in a direction from the tapered end to the wide end, and convex or flat in a plane through the contact surfaces of the positioning means. The third positioning means may be, for instance, ball-shaped, in which case the radius of curvature of the surface of the third positioning means is at least the same throughout at or near the contact surface. It may also have a cylindrical, or substantially ellipsoidal shape, or any shape in between. The contact surfaces of the first and second positioning means may be, and preferably are, formed as a hyperboloid, the hollow side of the hyperboloid facing the cone part.
Preferably, the first and second means have the same shape.
The shape and form of the contact surfaces (i.e. the surface engaging the cone part) and the relative position of the positioning means guide the cone part to an accurate position when a rectangular cone part is positioned in the positioning means, two of the corners being positioned by the first and second means and the opposite side being positioned by the third means.
Preferably, the normal vectors at the contact areas substantially cross each other at a point substantially lying on a line through the center of the rectangle and perpendicular to the rectangle. During positioning the contact areas exert forces on the cone part. These forces are substantially aligned along the normal vectors on the contact surfaces. When the normal vectors cross each other at a single point and the cone part is properly aligned, the total force exerted on the cone part is minimal and there is no torque exerted on the cone part.
The positioning system is particularly advantageous when used to position CRT""s having a tapered end with substantially rectangular cross-sections.
The method of manufacturing a CRT in accordance with the invention is characterized in that a cone part having a tapered end with a substantially rectangular cross-section and a wide end, the tapered end having two corner parts and an opposite side, is positioned in a positioning system which comprises a first and a second positioning means at positions substantially corresponding to two corners of a side of a rectangle, and a third positioning means at a position substantially corresponding to the middle of a side of the rectangle opposite the above-mentioned side, the first and second means having a saddle-shaped contact surface, of a generally convex shape in a direction from the tapered end to the wide end, and a generally concave shape in a direction towards a central part of the rectangle, the third positioning means having a generally convex shape in a direction from the tapered end to the wide end, and a generally convex or flat shape in a plane through the contact surfaces of the positioning means, the corner parts of the cone part being positioned by the first and second positioning means and the opposite side being positioned by the third positioning means.
The positioning system may also be used to position a xe2x80x98conventional CRTxe2x80x99, i.e. one having a tapered end with a substantially circular cross-section. The saddle shaped surface of the first and second means reduces the Hertzian forces that occur in the cone when it is positioned on the means. Especially for larger cones (larger than 29xe2x80x2), the weight of the cone may cause such large Hertzian forces (=contact forces when one body contacts another) that cracks appear. Reduction of these contact forces reduces the risk of cracks (which may lead to implosion or weakness of the cone).
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.